Leonardo remains notorious for not finishing work, this one he finished. He wasn't trained in fresco painting and tried his own method, which started to fail while he was still working on it. Ten years after he started this, he and Michelangelo were contracted to paint dueling frescoes of the Battle of Anghiari, and in the interval he still hadn't trained up, and ruined his surface. Michelangelo got called away by the Pope to begin the Sistine Chapel ceiling.
Sounds like some government contractors I know..................
Bless you for the history of art lesson. Really loved it.